Hi i have just started mapping for world at war zombies. Basically my map is in the middle of the woods with grass and what not. My first question is how do i make it so i cant walk through my trees without making no clip walls all around them.. cause that would be kind of annoying. A friend of mine told me to look in the prefabs folder but there are no trees in it. My second question is.. Why the hell are there patches of red in my grass? I got the skybox key and values working.. the map is lightened up a bit.. its just the ground is sort of red. My third question is.. How to i place these path nodes for the zombies.. specifically how to do it. I have set them up 8 units away from each other throughout the whole map but it says they are floating so i'm guessing they cant be in the air at all. My last question is how to make mist/fog so it has this creepy/scary look to the map.
If anyone can help me resolve these problems, please reply.

Thank you.

The error with red ground seems to be a result of not having a reflection probe in your map.

If you really don't want people to walk through your trees, then yes, you really will have to use noclip.

Path nodes should be slightly above the ground, not touching it though, and can probably be more like 32 units apart. They also shouldn't touch each other.

If you want to do fog as an effect in the level, as in, rolling, moving, cool looking fog, you'll need to follow an effects tutorial.

http://modsonline.com/Tutorials-read-555.html

If you simply want to add fog that increases from completely transparent closeup, to completely opaque at a certain distance, look into exp fog, and cull fog, which can be set in the .gsc file.

Thanks a lot for the help.. just one more question. What's a reflection probe?

If you right click in the 2D view, you can add a reflection probe. If you're familiar with mapping in the source engine, its the equivalent of an env_cubemap, basically when you compile reflections, it samples stuff around it to determine how it should process reflections. You should generally have one in each area of your map.